Light Bulbs in Parallel Circuits
In the last activity, we analyzed several different series circuits. In a series circuit, there is only one
complete pathway for the charge to travel. Here are the basic ideas of a series circuit:
•
•
•
•
•
•
one pathway only for charge to travel
the total or equivalent resistance is the summation of
individual resistances
we see voltage drops across resistors and light bulbs
these voltage drops = the voltage provided by the
batteries
power is conserved. Power to resistors and bulbs =
power consumed by them
ammeters or current probes can be placed at different
points in the circuit and will give you the same value.
Charge flow is identical throughout the circuit.
In a parallel configuration, the current splits into
BRANCHES at the NODES. The total current into a NODE equals to the total current leaving the
NODE. The amount of current going through a BRANCH (Called BRANCH CURRENT) is inversely
related to the resistance in the BRANCH. The battery provides an equal push or pressure
(VOLTAGE) to each branch.
Consequently, statements we can say about parallel circuits are a bit different than for series circuits.
In this activity, we will investigate parallel circuits using several different types of light bulbs.
Same Circuit
‘Light bulbs in parallel Lab’, p. 1
adg 3/27/2011
Part 1: Our basic Ohm’s Law
circuit. Let’s start here to get
baseline data and observations.
Construct the following circuit using
two batteries, a voltmeter, an
ammeter, a switch, and a round light
bulb.
You’ll have to move the voltmeter
around to get all of the readings.
Write down your observations and data here and also calculate the power delivered to the lamp:
Bulb brightness: _______________________________________ (describe how bright the bulb is)
Voltmeteracross battery Reading = __________________ Volts
Voltmeteracross lamp Reading = __________________ Volts
Ammeter Reading = ____________________ Amps
Powerfrom battery (P = Vacross battery I) = _________________ Watts
Powerto lamp (P = Vto lamp I) = __________________ Watts
Ideally, Power from battery = Power to lamp. It should be close but may not be exactly equal.
My data looks like this:
Bulb brightness: pretty bright
Voltmeteracross battery Reading = 2.65 V
Voltmeteracross lamp Reading = 2.44 V
Ammeter Reading = 0.292 A
Powerfrom battery (P = Vacross battery I) = 0.774 W
Powerto lamp (P = Vto lamp I) = 0.712 W
Ideally, Power from battery = Power to lamp. It should be close but may not be exactly equal.
0.774 W ≈ 0.712 W
‘Light bulbs in parallel Lab’, p. 2
adg 3/27/2011
Part 2: Two #14 Round Light Bulbs in Parallel
Now, we would like to make a circuit that looks like the following picture below.
The total current leaves the battery and splits up at the NODE along two BRANCHES (pathways).
Part of the current goes through the first bulb and the rest goes through the second light bulb. The
battery provides an equal push (voltage) to both branches.
The W stands for wires needed
One of the first things we can ascertain is the voltage across the different devices. Theory says that the
voltage across parallel devices should be the same.
NOTE: I have found that you should leave the switch off until you are ready to make your
measurements and then just turn the circuit on, take your readings and then turn it off while you move
your meters. The current drains the batteries and can give erroneous readings. Turn on, take
readings quickly and turn off.
First, put the voltmeter across the batteries, then across both lamps, next across lamp 1, and finally,
across lamp 2. Also, write down the current coming from the batteries (next page for the current
measurements).
Vbatteries = __________ V
Vacross both lamps = ___________ V
Vlamp 1 = __________ V
Vlamp 2 = __________ V
All of these values should theoretically be the same.
One thing you will notice is that it takes more wires in a parallel circuit than in a series circuit!
‘Light bulbs in parallel Lab’, p. 3
adg 3/27/2011
Now, let’s remove the voltmeter and insert an ammeter.
First, put the ammeter right after the batteries but before left node (see left picture). Write down your
ammeter reading. Then move the ammeter so it is measuring the branch current (middle picture) and
record this current reading. Finally, move the ammeter so it is measuring the branch current of the top
light bulb.
a) Current Reading Coming from Batteries (left pic): _____________ A
b) Current Reading going into bottom light bulb (middle pic): _____________ A
c) Current Reading going into bottom light bulb (right pic): _____________ A
Because we have two identical light bulbs, the current from the batteries should split into equal
amounts, with equal currents going to each light bulb. Consequently, the current from line a above =
current from line b + current from line c.
Is this pretty close for your circuit?
Go back to your page 2. You answered questions about the brightness, current, voltage, etc. using one
bulb only. Let’s try to make some comparisons with one light bulb and two light bulbs.
How did the brightness change with two light bulbs in parallel when compared to one light bulb
(p. 2)
How did the current (out of the batteries) change with two light bulbs in parallel when compared
to one light bulb (p. 2)
How did the voltage (across the batteries) with two light bulbs in parallel when compared to one
light bulb (p. 2)
‘Light bulbs in parallel Lab’, p. 4
adg 3/27/2011
Here are my thoughts over these last three questions…
My observations: It can be difficult to see changes in brightness. The circuit with one bulb was bright (p. 3 see my
observations). Two bulbs in parallel, ideally, should be the same brightness as with one bulb. This is definitely different
than when we compare one bulb and then two in series. When you placed two bulbs in series, their brightness levels were
much decreased than with one bulb (see pp. 3-4 of the series circuit lab).
You should see that current from the battery actually increases in the parallel circuit with two light bulbs. This is most
interesting. Again this is much different than with a series circuit. In a series circuit, adding additional resistors or light
bulbs in series makes the current decrease.
Your voltages should not really change that much. You might see a small change (a few tenths of a volt) depending upon
how fresh the batteries are.
Now the voltages stay about the same when we have one lamp vs. two lamps in parallel and the current
actually increases with two lamps in parallel. How is this possible? What must be happening to the
total or equivalent resistance if the current increases given no change in voltage?
This shows that the equivalent or effective resistance must decrease. Remember Ohm’s Law (V = IR). If V stays constant,
there is an inverse relationship between current and resistance. If current goes up, total resistance must decrease.
Consequently, Req in parallel ≠ Rlamp1 + Rlamp2. There must be a different relationship. As we discussed (or will discuss) in
class, the following relationship holds for resistors (or lamps) in parallel:
1
1
1
= +
Req R1 R2
As more resistors are added in parallel, the equivalent resistance drops. For the same voltage, more
charge flows from the battery with lesser resistance resulting in an increased current.
In the previous lab, we said that the round #14 light bulb has an approximate hot resistance of 10 Ω.
Using your equation above, what would the equivalent resistance be for two of these light bulbs
in parallel?
You should get 5 Ω. For two identical resistors or light bulbs the equivalent resistance is just half.
1/10 + 1/10 = 1/Req
2/10 = 1/Req
10/2 = Req = 5 Ohms
‘Light bulbs in parallel Lab’, p. 5
adg 3/27/2011
Let think about the series circuit again with two light bulbs.
Use your knowledge about series circuits to answer the
following question:
If you unscrew one of the bulbs, what happens to the
brightness of the other one?
Why is this true?
Now try the same thing with your parallel circuit. Unscrew the top bulb. What happens to the
bottom bulb? Try it the other way, too.
Offer an explanation…
Why might this property of parallel circuits be advantageous? Think
about holiday lights or appliances in your home…
‘Light bulbs in parallel Lab’, p. 6
adg 3/27/2011
Part 3: Three #14 Round Light Bulbs in Parallel
NOTE: I have found that you should leave the switch off until you are
ready to make your measurements and then just turn the circuit on, take
your readings and then turn it off while you move your meters. Leaving
the switch on can quickly drain the batteries and can give erroneous
readings. Turn on, take readings quickly and turn off.
First, build the circuit on the left and move your voltmeter around to the different locations. Then
remove the voltmeter and put the ammeter into the circuit at the locations indicated in the right-side
picture.
See the next page for spaces to put your voltmeter and ammeter readings.
‘Light bulbs in parallel Lab’, p. 7
adg 3/27/2011
Take the following readings:
Vbatteries = __________ V
Vlamp1 = ___________ V
Vlamp2 = ___________ V
Vlamp2 = ___________ V
All of these voltages should be similar although the voltage across the batteries may be a bit higher.
How did yours turn out?
Current coming from batteries = _____________ A
Current through lamp1 branch = _____________ A
Current through lamp2 branch = _____________ A
Current through lamp3 branch = _____________ A
Because all the bulbs are theoretically identical, all the branch currents should be close. And when you
add up these branch currents, it should be close to the current coming out of the batteries.
How did yours turn out?
My data:
V batteries = 1.9 V
V across all three bulbs = 1.46 V
Vlamp1 = 1.44 V
Vlamp2 = 1.47 V
Vlamp3 = 1.51 V
All are fairly close especially the voltage readings across each lamp.
Current coming from batteries = 0.617 A
Current through lamp1 branch = 0.248 A
Current through lamp2 branch = 0.245 A
Current through lamp3 branch = 0.236 A
If I add up my branch currents I get 0.729 A. This is a bit higher than the 0.617 A current coming from the batteries. This
is within the realm of experimental error.
‘Light bulbs in parallel Lab’, p. 8
adg 3/27/2011
One more circuit:
Part 4: Two different Light Bulbs in Parallel
In the last section of the previous lab on light bulbs in series, you wired up two different bulbs (one
round and one long). In this part, you will take these same bulbs and wire them in parallel.
Let’s go back to our observations of this part on the previous light bulb lab. Do you remember what
we saw for the brightness of each light bulb when placed in series?
a) In this part you will use two different light bulbs, one of the #14 round bulbs and then one of the
‘long’ (#48) light bulbs. Here are the specs on these two light bulbs. Resistances are approximate:
Bulb Type
Name
#14
#48
Round
Long
Approximate Resistance (Hot)
10 Ω
40 Ω
Approximate Resistance (Cold)
1.2 Ω
4.5 Ω
b) Build the circuit
c) When you have completed the wiring, turn on the switch.
d) Observe the light bulbs and you will see something very different than when we used the two
different bulbs in series). First off, what are your observations of the round bulb?
e) Observations of the long bulb?
‘Light bulbs in parallel Lab’, p. 9
adg 3/27/2011
f) From your observations, which bulb is emitting more power?
Well, let’s take some data. Put the voltmeter across the batteries, the round light bulb, and the long
light bulb separately and record the values.
Vbatteries = __________ V
Vround light bulb = ___________ V
Vlong light bulb = ___________ V
All of these voltages should be similar although the voltage across the batteries may be a bit higher.
Here are my voltages:
Vbatteries = 2.6 V
Vround light bulb = 2.2 V
Vlong light bulb = 2.3 V
Now find your currents. First put your ammeter in the circuit before the node to get the current
from the battery. Record the current. Then put your ammeter in each branch to get the current
going to each light bulb. Record these two currents, too.
Current coming from batteries = _____________ A
Current through round light bulb branch = _____________ A
Current through long light bulb branch = _____________ A
Here are my currents:
Current through round light bulb branch = 0.297 A
Current through long light bulb branch = 0.062 A
Current coming from batteries = 0.35 A
The branch currents sum = 0.359 A which is pretty close to the current from the batteries of 0.35.
Add your branch currents and see how close they were to your current from the batteries.
Were you close?
‘Light bulbs in parallel Lab’, p. 10
adg 3/27/2011
The branch current through the long light bulb is much smaller than the branch current through the
round bulb. This was not the case when we had two round bulbs in parallel. In that case, the battery
current split fairly equally between both branches.
Explain why the current from the batteries doesn’t split equally given the light bulb resistances
as found on page 8 of this lab…
The long bulb has about four times the resistance so its current should be about ¼ the current of the
round bulb. If I take the ratio:
longbulbcurrentbranch 0.062 A
=
= 0.21
roundbulbcurrentbranch 0.297 A
Let’s go back to your observations of the two light bulbs on the previous page. Didn’t the round bulb
look brighter than the long bulb? And isn’t this much different than we saw with the series circuit
where we couldn’t even see the round bulb light up? Can you explain this in terms of the power to
both bulbs in the parallel configuration vs. in the series configuration? You may need to go back to
your data in the series activity (pp. 6-8)
Power parallel-round bulb = V round light bulb*I round light bulb = _________ * __________ = _________ Watts
Power parallel-long bulb = V long light bulb*I long light bulb = _________ * __________ = _________ Watts
And from p. 7 of previous activity on series light bulbs:
Power series-round bulb = V round light bulb*I round light bulb = _________ * __________ = _________ Watts
Power series-long bulb = V long light bulb*I long light bulb = _________ * __________ = _________ Watts
Here’s all my data:
In parallel, I get 0.653 Watts for the round light bulb and 0.143 Watts for the long light bulb. This
helps explain why the round light bulb is much brighter in parallel.
In series, if I go back to the previous lab, the round light bulb had a power output of 0.00539 Watts (do
you remember that it didn’t light up) and the long light bulb’s power was 0.198 Watts.
OK, that’s about it for parallel circuits. For a pure parallel circuit, voltage is the same across each
resistor (or light bulb) and theoretically equals the voltage provide by the battery(ies). Current splits
along the different paths at the node and recombines later. Equivalent resistance actually increases and
follows the following relationship.
1
1
1
= +
Req R1 R2
Power is still conserved—power from the battery = power delivered to the resistors (light bulbs).
‘Light bulbs in parallel Lab’, p. 11
adg 3/27/2011